Add appropriate version contraint for mongoid.

`mongoid` 7 is the last major version that's compatible with `mongo`
driver 2.15 (which is the last version that's compatible with MongoDB
2, which we're stuck with for at least a few more weeks).

https://www.mongodb.com/docs/mongoid/current/reference/compatibility/

Hoping this also fixes the unexpected Dependabot behaviour in #592 where
it tries to downgrade `mongoid` all the way to version 1.

Gemfile

@@ -7,7 +7,7 @@ gem "gds-sso"
 gem "govuk_app_config"
 
 gem "mongo", "~> 2.15.1"
-gem "mongoid"
+gem "mongoid", "~> 7.5"
 
 group :development, :test do
   gem "brakeman"